The present invention relates to raster scan display systems, and particularly to an apparatus for generating one or more mobile cursors in such a system.
The operation of a graphics display system is enhanced by the provision of electronically generated cursors, one or more of which may be positioned on the display to identify part of a displayed image which a user wishes to alter in some way. For example, many systems provide a mobile alpha-numeric cursor with textual material to indicate a portion of text which is to be changed by the addition or deletion of alpha-numeric characters. In non-textual graphic images, other cursors may be used. A rectangular box may identify an image area to be panned or zoomed. Present systems may generate a number of different mobile cursors which enable a user to perform a variety of editing tasks on a displayed image.
In display systems incorporating analog television, cursors are generated by using electronic counters to calculate cursor dimensions and locations and insert their representative components into the video signal at the proper moment. The counters employ clock circuits which must be periodically synchronized with the generation of the television image. Between synchronization, the operational frequencies of the counting and the image generation circuits may drift apart, which can cause error in the location of the cursor and blur its outline. Rausch U.S. Pat. No. 3,778.058 Bushnell U.S. Pat. No. 3,793,483 exemplify this type of cursor generation system.
More modern raster-scan display systems produce clock pulses which are precisely phased with image display circuitry. Consequently, in these systems a cursor generator which utilizes a counting apparatus to position and draw a cursor can be inherently more accurate than the older analog systems. Doornink U.S. Pat. No. 4,190,834 for example, describes a circuit for producing a cross-hair cursor in synchronization with a raster-scan display. However, circuits of this type are not flexible enough to generate a large number of cursor types. If more cursors are required, the circuit must be expanded, or other circuits, dedicated to specific types of cursors, must be provided. The undesirable result is the proliferation of cursor generation circuitry. Therefore, it would be advantageous to have an apparatus which accurately generates a variety of cursors with a minimum of circuitry.
One approach to providing flexible, accurate cursor generation in a raster-scan graphics display would be to incorporate a separate cursor computation apparatus, including a dedicated cursor refresh storage buffer, into the display circuitry of the system. The output of the cursor buffer would be combined with that of the graphics image refresh buffer to produce a graphics image with the desired cursor symbols. However, as is well understood by those skilled in the art, provision of a separate buffer could involve the dedication of over 800K bits of random-access memory resulting in added system cost and complexity.
On the other hand, addition of cursor information into the image refresh storage buffer can affect the image information stored therein and further complicate access to it.
It is then evident that a need exists for an apparatus which will generate a variety of accurately drawn cursor symbols in a graphics display system without adding significantly to its hardware requirements or costs and without affecting its operation.